(1) Field of the Invention
The present invention pertains to subwavelength size, monopole, underwater sound radiators. The term subwavelength infers that overall radiator dimensions are a small fraction of one wavelength, the term monopole characterizing a radiation mode where the radiator's aggregate volume cyclically varies. A mechanism termed a driver cylically moves a radiating face or faces which are in contact with the medium to accomplish this volume change. Radiating face motion is facilitated by placing a compressible volume such as air, maintained at the same pressure as the surrounding medium, adjacent to interior radiating face surfaces and establishing pliant or jointed radiating face peripheral regions. The radiation impedance of isolated subwavelength size radiators is too reactive for the efficient radiation of sound. When radiation efficiency is important, subwavelength size radiators can be coupled to the medium with impedance matching horns, by assembly into closely packed arrays, and other techniques.
(2) Description of the Prior Art
It is usually desirable that individual subwavelength size radiators achieve as high a resistive and as low a reactive radiation impedance as possible. Pulsating spherical radiators are optimum for this purpose because they initiate spherical wavefronts at their radiating faces. However, they are difficult to achieve in a practical design. Gas-filled elastomeric spheres are probably the closes approach but their operating parameters are constrained by the resonance and other properties of the contained gas effectively interposing a depth dependent spring function between driver and radiating face. That spring function depending on its softness reduces bandwidth. Common radiating face shapes such as flat, hemispherical, and conical initiate substantially non-spherical wavefronts that evolve into spherical wavefronts by a nearfield diffractive process. The transposing of pressures and velocities in this process further contributes to the relatively high reactive radiation impedances and low efficiencies of many subwavelength sized radiators.
It is often desirable to amplify driver displacement to obtain a better match between driver and radiation impedance. This can be achieved by mechanical motion amplifiers, by large radiating faces, or combinations of both. Unfortunately, both methods involve introducing more structure into the oscillating process with associated problems of stiffness and weight. Weight adds to the reactive load to and from which energy must be transferred. Springs or equivalents are sometimes introduced in parallel with a driver to help control the reactive load but this increases mechanical Q and narrows bandwidth. Stiffness is equally important to a radiating face since it helps keep all sections of a face in phase. Conically shaped radiating faces can help solve these problems since they are inherently both stiff and light. They cyclically transpose the axial stresses imparted by the driver, that would have become bending stresses in a flat radiating plate, into circumferentially aligned tensional and compressional stresses. Unfortunately, as noted earlier, conical radiators do not approximate a pulsating sphere.
Other problems involve cooling applicable parts and supplying compressed air. The two problems can be related. The portion of the radiator interior not occupied by the driver is often filled with air the result being more air than is needed for the compressible volume function, air a part of which must be transferred with each change in depth. This air often serves to thermally and electrically insulate the driver. The electrical insulating function is desirable but can be replaced by various coatings and submergence in oil. The thermal insulating function is undesirable since it can both limit the strength of length of acoustic transmissions and subject the radiator to possible thermal damage.